1. Field of the Invention
The present invention relates to voltage regulation circuits and methods, an in particular, to DC-to-DC voltage conversion circuits and methods.
2. Related Art
In switching mode power supply systems, current limiting has two main purposes. The first is to protect circuit components from electrical overstress, and the second is to keep the system operating temperature within a specified range. Accordingly, if the current limiting is not accurate, over sized components and/or heat sinks are required to handle additional power dissipation. Alternatively, the power rating of the power supply must be limited.
However, operating temperature is not related so much to the peak output current as it is to the root mean square (RMS) current, which, in most designs, approximates the average current. In any event, regardless of the average current produced, the power devices (e.g., the switching transistors and output inductor) need to operate in the defined safe operating area (SOA). Accordingly, instantaneous current sensing methods, such as peak or valley current sensing, are generally used.
Unfortunately, the relationship between peak or valley current and the average current is not constant. Ripple variation affects the accuracy of using conventional average current sensing techniques to limit the output current. Factors affecting the ripple include input voltage, output voltage, filter inductance and switching frequency. Input voltage and switching frequency variations are often a requirement and offering higher tolerances may provide a competitive advantage. Therefore, there is incentive to allow output current ripple to vary by more than 50%, thereby often requiring an overdesigned system, including more expensive switching transistors.
Accordingly, there is a need for a current limiting technique that delivers better accuracy by reducing sensitivity to the voltages and the circuit elements and parameters affecting ripple, while maintaining instantaneous SOA protection.